The present invention relates generally to a mixer for preparing silver halide emulsions for photographic use, and, more particularly, to a mixer that facilitates separate control of reactant dispersion (micromixing) and bulk circulation in the precipitation reactor (macromixing).
Silver halide grains can be formed by the double decomposition reaction of a water soluble silver salt solution and a water soluble halide solution. For photographic use, the goal is to produce silver halide grains of narrow grain size distribution because small grains of uniform size produce higher quality photographs than large grains or randomly distributed grains. In producing silver halide grains, two actions occur. First, there is micromixing where the individually introduced silver and halide solutions react to form silver halide grains. Second, there is macromixing, where the silver halide grains are circulated in the bulk liquid. Conventional mixers present several problems such as construction complexity, difficulty of cleaning, and air entrainment.
A mixer is disclosed in U.S. Pat. No. 4,289,733 that addresses several previously existing problems, and also points out the problem with conventional mixers that the degree of mixing within the mixing device and the circulation of the bulk liquid are both dependent on the rotation of the mixing device. However, the mixer disclosed therein does not completely solve this problem. Careful characterization of this prior mixing device reveals that the degree of mixing within the mixing device (micromixing) and the circulation of the bulk liquid through the mixing device (macromixing) cannot be independently controlled. Therefore, in scale-up operations where both micromixing and macromixing are key parameters that must be scaled, a problem still exists with prior mixing devices. It is desirable to have a mixer with improved robustness (i.e., reduced variability in particle size, size distribution, morphology and sensitometry) and scalability at all scales, while maintaining or improving photographic performance.
The present invention is directed to overcoming one or more of the problems set forth above. According to one aspect of the present invention, a scalable apparatus for preparing silver halide grains, comprises a vertically oriented draft tube, a bottom impeller positioned in the draft tube, and a top impeller positioned in the draft tube above the first impeller and spaced therefrom a distance sufficient for independent operation.
The mixer facilitates separate control of reactant dispersion (micromixing) and bulk circulation in the precipitation reactor (macromixing). This is achieved by the use of a draft tube, which houses two different impellers. The bottom impeller is a flat blade turbine (FBT) and is used to efficiently disperse the reactants, which are added at the bottom of the draft tube. The top impeller is a pitched blade turbine (PBT) and is used to circulate the bulk reactor fluid through the draft tube in an upward direction providing a narrow circulation time distribution through the reaction zone. Appropriate baffling is used to avoid vortexing and air entrainment. The two impellers are placed at a distance such that independent operation is obtained. This independent operation and the simplicity of its geometry are key features that make this mixer well suited in the scale-up of silver halide precipitation processes.
The impellers are placed at a distance that is shown to facilitate independent operation. Then, for scale up operations both the mixer rotation speed and the pitch angle of the upper impeller can be changed to simultaneously match micromixing and macromixing. For instance, if the only important parameter to be scaled is the bulk circulation, then, geometric similarity is used from one scale to another and the rotation speed is kept the same. On the other hand, if both reactant dispersal and bulk circulation are important parameters, then, geometric similarity is used, the rotation speed is changed to match power dissipation from one scale to another and the pitch angle of the upper impeller is changed in order to also match the bulk circulation from one scale to another.
In addition to the scalability, another advantage of the mixer is related to the morphology of cubic AgCl grains. With conventional radial mixing devices the cubicity of the AgCl grains is dependent on the mixing speed. With the present invention, mixing sensitivity of AgCl grain cubicity is significantly lower and not dependent on impeller speed, meaning that scale up of such grains should be easier. Another advantage is that, in the absence of antifoggants, R-typing is lower compared to conventional radial mixing devices. Another advantage of the new mixing device is that the reactant introduction process is very robust because of the efficient dispersing power of the lower impeller, and no reactant distribution or spreading is necessary.